In order to possess a flexible characteristic, the current flexible displays use a flexible substrate, such as a plastic substrate or a polymer substrate, to replace a glass substrate. Since the chemical-resistant property of a flexible substrate is poor, a flexible substrate is easily damaged during an etching process. A dielectric protective layer is typically added to obviate damages to the device. The dielectric protective layer may be a silicon oxide, aluminum oxide or silicon nitride type of inorganic material dielectric layer.
Asides from protecting the flexible substrate, the inorganic material dielectric layer may also have the function of blocking moisture and oxygen. For example, when the subsequently applied device is a photovoltaic cell or an organic light-emitting diode (OLED), this type of devices requires better moisture-blocking and air-blocking effect. Currently, such an effect is achieved via the stacking of multiple layers of the dielectric protective layer.
If the flexible substrate is formed by a thin film fabrication process with a sheet-to-sheet method, the phenomenon of residual stress often occurs. Typically, the residual stress that is generated on the dielectric protective layer of the flexible substrate is divided into two parts: intrinsic stress and thermal stress, wherein the intrinsic stress is further divided into compressive stress and tensile stress. Irrespective to the stress being a compressive stress or a tensile stress, the flexible substrate may become warped when the flexible substrate is removed from a glass carrier. Hence, the risk of broken lead/line and misalignment in the photolithography process is increased.